1. Field of the Invention
The present invention relates to correction technology for improving image formation defects caused by defective recording elements in a recording head having a plurality of recording elements, such as an inkjet head, and more particularly, to a parameter selection chart suitable for determining a parameter to be used in correction processing, and a parameter determination method, a parameter determination apparatus and an image forming apparatus using the parameter selection chart.
2. Description of the Related Art
In image formation by an inkjet method, when the inkjet head starts to be used, nozzles which are in a state of ejection failure due to blockages or breakdown occur. In particular, in the case of image formation by a single pass method, the ejection failure nozzle locations are perceived as white stripes and therefore the correction is required. There have been a large number of suggestions thus far in respect of ejection failure correction technology (e.g. Japanese patent application publication No. 2008-168592).
FIG. 29 shows a schematic diagram of the basic concept of ejection failure correction. When an ejection failure nozzle occurs in a print head 800, a white stripe occurs in the image formation region corresponding to this nozzle. Therefore, when correcting ejection failure, the visibility of the white stripe is reduced by increasing the density of image formation performed by nozzles which are adjacent to the ejection failure nozzle (referred to as “ejection failure correction nozzles”). There are various methods for increasing the density of image formation by ejection failure correction nozzles, for example: (1) correcting the output image, (2) strengthening the ejection signal to make the diameter of the ejected dots larger, and so on.
(1) Method of Correcting Output Image
If the image density for image formation of the surrounding area is taken as Ddefault, then it is possible to increase the image formation density of the ejection failure correction nozzles and reduce the visibility of a white stripe, by setting the image density for the ejection failure correction nozzles to DNo Print (>Ddefault). The ratio between these image densities can be defined as the ejection failure correction nozzle image density increase Pdensity.
(2) Method of Strengthening Ejection Signal and Enlarging Ejection Dot Diameter
If the dot diameter for image formation of the surrounding area is taken as Rdefault, then it is possible to increase the dot diameter of the ejection failure correction nozzles and reduce the visibility of a white stripe, by setting the dot diameter for the ejection failure correction nozzles to RNo Print (>Rdefault) The ratio between these dot diameters can be defined as the ejection failure correction nozzle dot diameter increase Pdot.
In the present specification, the amount of strengthening of image formation by the ejection failure correction nozzles, such as the ejection failure correction nozzle image density increase Pdensity and the ejection failure correction nozzle dot diameter increase Pdot in the two typical examples described above, and the amount of correction similar to this, are defined generally as an ejection failure correction parameter P.
If the ejection failure correction parameter P is too large, then a black stripe is formed due to over-correction, and if the ejection failure correction parameter P is too weak, then a white stripe is formed due to under-correction. Therefore, technology for finding the optimal value of P is required.
Overview of Method Disclosed in Japanese Patent Application Publication No. 2008-168592
In Japanese patent application publication No. 2008-168592, the ejection failure correction parameter is calculated on the basis of scan data produced by an optical reading device from an optimal value selection chart. FIG. 30 shows an overview of a measurement procedure disclosed in Japanese patent application publication No. 2008-168592. A correction amount R for density non-uniformity caused by ejection failure is calculated by the following procedure.
[1] A uniform image (“normal test pattern”) is output in which an image is formed on a prescribed region of a sheet of paper at uniform densities based on respective measurement tones corresponding to a plurality of different densities, and this image is scanned to measure the average tone value Ra. [2] A plurality of nozzles are disabled for ejection, a uniform image similar to that described above (“omitted nozzle test pattern”) is output, and this image is scanned to measure the average tone value Rb. [3] The ratio Ra/Rb is calculated, and this is set as the ejection failure correction parameter.
However, in the method described in Japanese patent application publication No. 2008-168592, the measurement accuracy of the ejection failure correction parameter is expected to decline due to the following factors.
<1> Human visual characteristics are not taken into account. There is a large disparity between the tones read by a scanner and human visual characteristics. Moreover, in the method described in Japanese patent application publication No. 2008-168592, measurement results vary with the reading resolution of the scanner. The measurement accuracy declines due to the combination of these.<2> No evaluation is carried out after the ejection failure correction parameter has been applied. The image formation by ejection failure correction nozzles adjacent to an ejection failure nozzle is of a tone darker than the surrounding uniform image area. Therefore, the amount of change in the contrast, in other words, in the visibility of the ejection failure correction results, becomes greater with respect to the ejection failure correction parameter. Moreover, the visibility varies greatly due to various factors, such as the position error, dot diameter variation, landing interference, and the like, in the vicinity of the ejection failure nozzle.
Furthermore, with a single pass method, it is common to employ a composition of the print head 800 in which a plurality of head modules 802 having the same design are arranged in a direction perpendicular to the direction of conveyance of the paper 820, as shown in FIG. 29. If the same ejection failure correction parameter is applied to each of the head modules 802 as shown in FIG. 31A, then ideally the same correction result is obtained, but in actual practice, the visibility of the correction results varies between the head modules, as shown in FIG. 31B.
<3> If the test pattern (chart) is output by an inkjet printer, then image non-uniformities occur due to the effects of positional error, ejection non-uniformities, and other factors. This is a cause of error in the measurement of the ejection failure correction parameter on the basis of the output results of the test pattern. In a method which measures the average tone value as in the procedure described in Japanese patent application publication No. 2008-168592, it is not possible to eliminate the effects of image non-uniformities entirely.